Measuring the velocity of a flow is needed in many applications, e.g. when determining the streamflow in an open channel. Such measurements are for instance important in the realm of irrigation, drinking water supply, hydroelectric power production, flood control, reservoir control, sewage systems, preservation of ecosystems, etc. Streamflows can occur in many different types of structures, natural courses, artificial channels, furrows etc. All of them are open-channel flows, i.e. flows having a free surface.
There are many different systems available for measuring the velocity. Systems which are image based possess the advantage that there is no need of expensive installations and they offer more flexibility than other non-intrusive measurements systems.
A long established technique for velocity measurements is the technique of Particle Image Velocimetry (PIV); see e.g. R. J. Adrian, 1991, “Particle-imaging techniques for experimental fluid mechanics.” Annual Review of Fluid Mechanics 23, 261-304. Ply is since the work of Ichiro Fujita, Marian Muste, and Anton Kruger (1998, “Large-scale particle image velocimetry for flow analysis in hydraulic engineering applications.” Journal of Hydraulic Research 36 (3): 397-414) also known of being applied to large scale free surface flows of flumes or open channels. This flavor of PIV is known to the hydraulic research and engineering community as Large Scale PIV (LSPIV), see e.g.                Muste, M., I. Fujita, and A. Hauet. 2008. “Large-Scale Particle Image Velocimetry for Measurements in Riverine Environments.” Water Resources Research 44 (4), and Muste, M., H.-C. Ho, and D. Kim. 2011. “Considerations on Direct Stream Flow Measurements Using Video Imagery: Outlook and Research Needs.” Journal of Hydro-Environment Research 5 (4): 289-300.        
Recent examples of LSPIV being applied to river flows are described e.g. in                patent application WO 2014/013064 A1,        Kim, Y., M. Muste, A. Hauet, W. F. Krajewski, A. Kruger, and A. Bradley. 2008. “Stream Discharge Using Mobile Large-Scale Particle Image Velocimetry: A Proof of Concept.” Water Resources Research 44 (9),        Dramais, Guillaume, Jerome Le Coz, Benoît Camenen, and Alexandre Hauet. 2011. “Advantages of a Mobile LSPIV Method for Measuring Flood Discharges and Improving Stage-discharge Curves.” Journal of Hydro-Environment Research 5 (4): 301-12, and        Tsubaki, Ryota, Ichiro Fujita, and Shiho Tsutsumi. 2011. “Measurement of the Flood Discharge of a Small-Sized River Using an Existing Digital Video Recording System.” Journal of Hydro-Environment Research 5 (4): 313-21.        
All these mentioned PIV methods have in common that they require having a well detectable flow tracer of some kind. However, the use of natural or artificial tracers possesses some complications to practically measure the velocity, since the tracers are not generally present and/or cannot be added permanently and continuously.
In the method disclosed in WO 2014/013064 A1 the velocity of the water surface is determined by subtracting two images to obtain a composite image. Subsequently, a suitable threshold is chosen to suppress the non-moving zones and a PIV analysis is performed to determine the velocity of some tracers. Since the displacement of the tracers is determined within the same composite image, their direction of movement is indefinite.
The depth of water can also be estimated using images. In WO 2014/013064 A1 it is proposed to analyze pixel colorimeter to determine the water level. However, using pixel colorimeter can possess some difficulties for analyzing images with poor light conditions. The method described in WO 2014/013064 A1 also needs at least one visual reference object being partially immersed in water, which constrains the places where the system can be installed. In addition, at least 6 reference points which have to be geo-referenced are needed for the camera calibration, in which the external parameters (i.e. position and orientation and at least the internal parameter focal distance) are obtained.